Agricultural ammonia emissions in China: Reconciling bottom-up and top-down estimates

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Abstract

Current estimates of agricultural ammonia (NH<sub>3</sub>) emissions in China differ by more than a factor of 2, hindering our understanding of their environmental consequences. Here we apply both bottom-up statistical and top-down inversion methods to quantify NH<sub>3</sub> emissions from agriculture in China for the year 2008. We first assimilate satellite observations of NH<sub>3</sub> column concentration from the Tropospheric Emission Spectrometer (TES) using the GEOS-Chem adjoint model to optimize Chinese anthropogenic NH<sub>3</sub> emissions at the 1/2°&amp;thinsp;&amp;times;&amp;thinsp;2/3° horizontal resolution for March&amp;ndash;October 2008. Optimized emissions show a strong summer peak with emissions about 50&amp;thinsp;% higher in summer than spring and fall, which is underestimated in current bottom-up NH<sub>3</sub> emission estimates. To reconcile the latter with the top-down results, we revisit the processes of agricultural NH<sub>3</sub> emissions, and develop an improved bottom-up inventory of Chinese NH<sub>3</sub> emissions from fertilizer application and livestock waste at the 1/2°&amp;thinsp;&amp;times;&amp;thinsp;2/3° resolution. Our bottom-up emission inventory includes more detailed information on crop-specific fertilizer application practices and better accounts for meteorological modulation of NH<sub>3</sub> emission factors in China. We find that annual anthropogenic NH<sub>3</sub> emissions are 11.7&amp;thinsp;Tg for 2008 with 5.05&amp;thinsp;Tg from fertilizer application and 5.31&amp;thinsp;Tg from livestock waste. The two sources together account for 88&amp;thinsp;% of total anthropogenic NH<sub>3</sub> emissions in China. Our bottom-up emission estimates also show a distinct seasonality peaking in summer, consistent with top-down results from the satellite-based inversion. Further evaluations using surface network measurements show that the model driven by our bottom-up emissions well reproduces the observed spatial and seasonal variations of NH<sub>3</sub> gas concentrations and ammonium (NH<sub>4</sub><sup>+</sup>) wet deposition fluxes over China, providing additional credibility to the improvements we have made to our agricultural NH<sub>3</sub> emission inventory.

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Zhang, L., Chen, Y., Zhao, Y., Henze, D. K., Zhu, L., Song, Y., … Huang, B. (2018). Agricultural ammonia emissions in China: Reconciling bottom-up and top-down estimates. Atmospheric Chemistry and Physics, 18(1), 339–355. https://doi.org/10.5194/acp-18-339-2018

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